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My feeling is that it's because 3-3 is a poor basis to extend from. Once the 3-3 stone is there, the corner is solidly yours; but in exchange, you have less to build upon because the position is so low.

The san-ren-sei point in the middle of the lower part of the board, and a point around white's 3-3 point stone in the lower left hand corner are "miai." That is if White takes one, black will take the other.

Let's say white closes his corner in the lower left by taking the 5-4 point, as he did in the upper left. Black then takes the san-ren-sei point in the lower middle side, which is a perfect extension from his shimari in the lower right.

If White takes the san-ren-sei point, Black has a perfect invasion point of White's moyo at the 4-4 point in the lower left, as shown in the diagram above.

Either way, Black gets the LAST "big point," thereby retaining the advantage of the first move (even after komi is factored in).

The thought process on that looks to be around the general strength of playing at the hoshi. If white were to play there, black would have an opening to take a third corner, but by playing on the third line, white made it harder for black to take the corner from white.